A computer keyboard is one of the essential devices in the modern lives. By pressing one or more keys of the keyboard, a corresponding character or a corresponding command is issued to a computer. Generally, various symbols are marked on a keycap of the key for facilitating the user to input the character or the command. For example, these symbols include an English alphabet, a phonetic notation, a punctuation mark, and so on. In other words, the symbols marked on different keys are different. For complying with different languages or different input methods, four or five symbols with different properties are possibly marked on one key. Consequently, it is difficult to recognize the symbols.
Nowadays, some kinds of keyboards are equipped with light-emitting elements and light guide plates. The light beams emitted by the light-emitting elements are scattered to the regions around the keys or the light beams are transmitted through the keycaps of the keys. Consequently, the users can clearly view the symbols on the keys when the keyboards are used or operated in a dim or dark environment. However, since the keyboard has so many keys and the lighting area is very large, the white light or the single color light is used in the illuminated keyboard. Under this circumstance, the symbols with different properties and on the key are difficultly highlighted. That is, the complicated symbols usually confuse the user. Therefore, the conventional technology needs to be further improved.